They are not really complex, its just a matter of hooking up a water line and most provide the sink adaptor, or it is inexpensive to get at a hardware store (I guess they are home improvement stores now, I'm showing my age) Check these out: http://www.thefilterguys.biz/ro_di_systems.htm , A little more money than EBAY but offer better quality and clear filter housings so you can check on the filters easier. They are also pretty much plug and play with no need to buy any additional parts.
It really depends on how you have it hooked up, I would get the pressure guage for sure, the flush valve is handy but not needed, I would not bother with the bypass valve, the rest depends on wether you are going to use as auto top off or just making it and carrying it to the tank.